In modern life, house prices and rents increase rapidly, and thus low-income group would have to choose small-sized houses with a single bedroom. Normally, the usable floor area of such kind of house is only dozen to twenties square meters, and thus a bed would occupy a half of the area. It is inconvenient to walk around the bed. In this connection, a multi-purpose furniture is desirable, which could be converted to a bed for sleeping, or be converted to a sofa and a coffee table when not used for sleeping. Currently, the conversion operation for most of the existing furniture, such as sofa beds in the market is inconvenient. Meanwhile, such furniture generally is provided with springs or oil-pressure pumps, which needs a large occupation area and is generally very expensive.